docs: def flattened(self):
          """return flattened data ``(x, f)`` such that for the sweep through
          coordinate ``i`` we have for data point ``j`` that ``f[i][j] == func(x[i][j])``
  
          """
          flatx = {}
          flatf = {}
          for i in self.res:
              if isinstance(i, int):
                  flatx[i] = []
                  flatf[i] = []
                  for x in sorted(self.res[i]):
                      for d in sorted(self.res[i][x]):
                          flatx[i].append(x)
                          flatf[i].append(d)
          return flatx, flatf